The invention relates generally to mobile networks, and particularly to transmission to be implemented in their fixed network part. A fixed network in this context denotes that part of a mobile network which extends in the uplink direction from the base stations. Even though a fixed network is referred to in this context, it is to be noted that this fixed network or part thereof may be implemented with radio links, for example.
To illustrate the typical architecture of a mobile network, FIG. 1 shows the structure of the known GSM mobile communications system (Global System for Mobile Communications), using abbreviations known from the context of the GSM system. The system comprises several open interfaces. The transactions relating to crossing of interfaces have been defined in the standards, in which context the operations to be carried out between the interfaces have also been largely defined. The network subsystem (NSS) of the GSM system comprises a mobile services switching centre (MSC) through whose system interface the mobile network is connected to other networks, such as a public switched telephone network (PSTN), an integrated services digital network (ISDN), other mobile networks (Public Land Mobile Networks PLMN), and packet switched public data networks (PSPDN) and circuit switched public data networks (CSPDN). The network subsystem is connected across the A interface to a base station subsystem (BSS) comprising base station controllers (BSC), each controlling the base transceiver stations (BTS) connected to them. The interface between the base station controller and the base stations connected thereto is the Abis interface. The base stations, on the other hand, are in radio communication with mobile stations across the radio interface.
The GSM network is adapted to other networks by means of the interworking function (IWF) of the mobile services switching centre. On the other hand, the mobile services switching centre is connected to the base station controllers with PCM trunk lines crossing the A interface. The tasks of the mobile services switching centre include call control, control of the base station system, handling of charging and statistical data, and signalling in the direction of the A interface and the system interface.
The tasks of the base station controller include, inter alia, the selection of the radio channel between the controller and a mobile station MS. For selecting the channel, the base station controller must have information on the radio channels and the interference levels on the idle channels. The base station controller performs mapping from the radio channel onto the PCM time slot of the link between the base station and the base station controller (i.e., onto a channel of the link). The establishment of the connection will be described in closer detail in the following.
The base station controller BSC schematically shown in FIG. 2 comprises trunk interfaces 21 and 22 through which the BSC is connected to the mobile services switching centre across the A interface on the one hand and to the base stations across the Abis interface on the other hand. The transcoder and rate adaptation unit TRAU forms part of the base station system and may be incorporated into the base station controller or the mobile services switching centre. For this reason, the unit is shown in broken line in FIG. 2. The transcoders convert speech from a digital format to another, for example convert the 64 kbit/s PCM signals arriving from the mobile services switching centre across the A interface into 13 kbit/s coded speech signals to be conveyed to the base station, and vice versa. Data rate adaptation is performed between the speed 64 kbit/s and the speed 3.6, 6, or 12 kbit/s. In a data application, the data does not pass through the transcoder.
The base station controller configures, allocates and controls the downlink circuits. It also controls the switching circuits of the base station via a PCM signalling link, thus enabling effective utilization of PCM time slots. In other words, a branching unit at a base station, which is controlled by the base station controller, connects the transmitter/receivers to PCM links. Said branching unit transfers the content of a PCM time slot to the transmitter (or forwards it to the other base stations if the base stations are chained) and adds the content of the receive time slot to the PCM time slot in the reverse transmission direction. Hence, the base station controller establishes and releases the connections for the mobile station. Multiplexing of the connections from the base stations to the PCM link(s) crossing the A interface is carried out in switching matrix 23, as is the reverse operation.
The layer 1 physical interface between the base station BTS and the base station controller BSC is in this example a 2048 kbit/s PCM line, i.e. comprises 32 64 kbit/s time slots (=2048 kbit/s). The base stations are fully under the control of the base station controller. The base stations mainly comprise transmitter/receivers providing a radio interface towards the mobile station. Four full-rate traffic channels arriving via the radio interface can be multi-plexed into one 64 kbit/s PCM channel between the base station controller and the base station, and hence the speed of one speech/data channel over this link is 16 kbit/s. Hence, one 64 kbit/s PCM link may transfer four speech/data connections.
FIG. 1 also shows the transfer rates used in the GSM system. The mobile station MS transmits speech data across the radio interface on the radio channel for example at the standard rate 13 kbit/s. The base station receives the data of the traffic channel and switches it to the 64 kbit/s time slot of the PCM link. Three other traffic channels of the same carrier are also located in the same time slot (i.e., channel), and hence the transfer rate per connection is 16 kbit/s, as stated previously. The transcoder/rate adaptation unit TRAU converts the encoded digital information to the rate 64 kbit/s, and at this rate the data is transferred to the mobile services switching centre. If the transcoder/rate adaptation unit is incorporated into the mobile services switching centre, maximum advantage is gained from compressed speech in data transmission.
In mobile networks of the above kind, the coverage area of the base stations has conventionally been wide, and thus there have been no great variations in the number of users roaming in the area of an individual base station, but the average traffic on the base station has been relatively uniform. In other words, the radio path has been concentrative (less channels in the radio path than users in the base station area). However, with the increase in the number of users, there has been a shift to using base stations having ever smaller coverage areas. With reduced cell size, the relative traffic fluctuations at the base station gain in magnitude. While the base station network continues to increase in density, it has become a problem how new base stations can be added to the network with maximum rapidity, flexibility and economy (with maximum utilization of the existing transmission capacity). This problem will be described in detail in the following in light of the current technology and the requirements to be set on the network.
Present-day mobile networks as a rule have fixed channels from the base stations to the base station controller, and such channels have been allocated in accordance with the overall radio path capacity of the base station. For example in TDMA-type (Time Division Multiple Access) radio systems, the time slot of the radio path is directly bound to the time slot of the transmission network (or part thereof. The transmission capacity of the fixed network is additionally needed for signalling between the base station and the base station controller and for network management.
The use of permanently allocated transmission channels (time slots) between the base station and the base station controller is economical for instance in cellular GSM and DCS systems in which one radio channel requires a capacity of 16 kbit/s in each transmission direction. On the other hand, the capacity required for example in the DECT system is normally 32 kbit/s for speech and often even more for data transmission. The required capacity may also be different in the different transmission directions. In that case, preparedness for worst-case capacity will waste a considerable amount of transmission capacity in normal use.
As for the fixed network, the utilization rate of transmission links has been enhanced by using cross-connects. These have enabled combination of 2048 kbit/s or 1544 kbit/s bit streams arriving from different base stations and arrangement of the 64 kbit/s time slots or parts thereof reserved by them. The cross-connects have a switching database of their own, which is defined in association with network installation and which can be changed if necessary for instance through operation and maintenance. At the same time when switching changes are made in a given cross-connect, the time slots reserved by the base stations and base station controllers must possibly also be changed and changes made in the other cross-connects as well. The maintenance of switching data in different equipments is thus a difficult task, particularly in complex networks.
When the coverage areas of base stations are reduced and their number increases, transmission networks have nevertheless simultaneously become ever more complex. On account of such a development, the maintenance of fixed network switching data in connection with mobile networks has become more difficult.
The mobile network operators should also be able to provide their customers with new services as simply and economically as possible. The novel mobile communications systems (such as the DECT) enable for example packet-switched transmission and also transmission performed at different rates in different directions. Implementing these functionalities economically and without wasting transmission capacity requires novel technical solutions.
The mobile network operator also needs to use the same transmission links for implementing different services. These transmission links are typically in accordance with the international standards and have the rate of either 2048 kbit/s (the E1 signal used in Europe) or 1544 kbit/s (the T1 signal used in the U.S.A.). Hence, it must be possible to allocate time slots from the same link for other uses than for the transmission of a specific mobile network.
As the transmission between the base stations and the base station controller already at present forms an essential part of the costs of the mobile network, it is obvious that with the increase in number and density of base stations, the significance of efficient and economical transmission solutions is emphasized even more. The rapid set-up of new base stations and the maintenance and flexibility of the transmission network have become key factors that are striven for in mobile networks.
The essential feature in the current network solutions is the need to carry out detailed network planning on time slot level. The requisite careful network planning on time slot level makes changes and additions to the network difficult and complex. Therefore, the present network solutions do not offer a suitable basis for achieving the aims set forth above.
It is an object of the invention to provide a solution wherewith transmission in the fixed part of a mobile network can be implemented in such a manner that the above aims are achieved more simply and cost-effectively than heretofore.
This object is achieved with the solutions defined in the independent claims.
The idea of the invention is to allocate the transmission capacity of the fixed part of the mobile network flexibly in accordance with the capacity needs in the radio path and to combine base stations on the transmission link between the base stations and the base station controller at least at one connection point where the digital signals from the base stations are logically combined in such a way that in each time slot or part thereof, only the signal to which said time slot or part thereof is allocated will get through.
With the solution of the invention, transmission in a fixed network is made concentrative and no intelligent cross-connects are needed in the network. The transmission network need not know in which time slot each base station is transmitting, but the time slot utilization information is located at the base stations and base station controller only. In practice, the intelligence can reside at the base station controller only, which grants the base stations permission to use time slots or parts thereof.
Since in this way transmission will be made transparent for the fixed network, transmission network planning on time slot level is eliminated. Neither do the cross-connects need any switching databases, and hence there is no need of operation and maintenance to change the switching.
Static control of transmission (in which a specific dedicated set of time slots in the frame of the common transmission link is allocated to each base station) already affords the above advantages. If, moreover, time slots (or parts thereof) are allocated fully dynamically (all transmission time slots in the transmission frame of the common transmission link are available to each base station), considerable savings in transmission capacity are achieved in many cases.
Since the concentration can be implemented in the transmission between the base station and the base station controller, considerable savings are achieved as the number of transmission links and switching ports at the base station controller can be smaller than heretofore.
On account of the above factors, new base stations can be taken into use and the network capacity can be increased more flexibly and cost-effectively than hitherto. Also introducing new services in the networks and transfer of other services using the transmission links of the mobile network is facilitated.